The present invention relates to the field of broadband communications. More specifically, the present invention relates to methods and apparatus for calibrating and controlling power output levels in a broadband communication system.
Hybrid fiber coaxial (HFC) networks are used by cable companies to provide services such as analog and/or digital video, video on demand, data communications, and telephony services to the home. These networks utilize a headend system, often referred to as a cable router or CMTS (cable modem termination system), to send information through an RF link from the cable plant to the end user. The HFC network requires that the cable router be capable of operating over a broad range of downstream frequencies, such as 50-860 MHz, and a wide range of output power levels, such as +48 to +61 dBmV.
These requirements present a challenge in the design of the cable router upconverter. The upconverter is the RF interface between the cable router and the HFC network. The HFC network relies on the upconverter to handle an absolute output power accuracy of +/−1 dB over the specified operating range. This accuracy and stability is required to properly set up the multi-channel network. If set incorrectly, the upconverter could disrupt the quality of cable services offered to the end user. It is difficult to calibrate and maintain this output power accuracy over a broadband or multi-octave range of operating frequencies.
At one frequency or a narrow band of frequencies, calibrating and setting output power is a fairly common task. Completing this task over a broadband range of frequencies with the required accuracy is more difficult. Calibration times get longer, more memory space is required to store calibration values, and the upconverter gain response over frequency must be taken into consideration.
For example, a common way to calibrate and set output power is via a brute force method. At factory calibration the upconverter is set to a desired frequency of operation and the gain of the upconverter is adjusted until desired output level is obtained. This gain setting is then stored in a lookup table and can be recalled at a later time. This method is acceptable for a narrow band of frequencies and power levels. As the number of frequencies increase, the upconverter calibration time increases, which tends to slow down factory output. The increased number of gain settings that need to be stored also increases the need for more memory space on the product. The number of points for calibration and storage grow as various output power levels at a wider range of frequencies are required. The matrix of calibration points versus output level versus frequency can get quite large while trying to maintain the required absolute accuracy at the upconverter output.
Improvements to this prior art calibration method include adding more sophisticated circuits to improve performance. For example, a power detector can be used near the output of the upconverter to convert RF power to a dc voltage. This dc voltage can then be used in software or hardware closed control loop to set the output power level at the appropriate level. While this can solve the problem of maintaining and setting the output power level at a discrete or narrow band of frequencies, it does not work as well in a wideband application as the frequency response of the upconverter and the sensitivity of the detector will vary with frequency of operation.
In addition, once the upconverter has been calibrated (e.g., at the factory or in the field), the accuracy of the output power level of the upconverter must be maintained or controlled during operation. There are many prior art approaches for maintaining accuracy of the output power level of the upconverter. For example, an open loop power control circuit may be used. Such open loop circuits suffer from significant temperature drift when using an analog attenuator or require a broadband digital attenuator with step size equal to that of the transmitter output power step size. This greatly reduces the candidate parts available which meet the desired power output resolution and all other specification requirements over a large frequency range. Further, analog or continuous attenuators typically have a nonlinear attenuation versus control signal response making them difficult to represent for calibration over the power level range of the transmitter.
In addition, channel filtering is not possible in broadband RF circuits due to the channel linearity requirements and the number of channels that have to be realized. Switched filtering may be used to bandpass filter the selected output frequency. However, there will always be a range of unfiltered frequencies around the carrier. The output carrier-to-noise ratio (C/N) for such a transmitter will be determined by the input level to the frequency tuned mixer (upconverter).
FIG. 1 shows a prior art example of open loop control of output power levels at an intermediate frequency (IF) using an analog or digital attenuator 10 before the frequency tuned mixer/upconverter 12. The attenuator 10 is used to vary the power level of an IF signal input into the mixer 12. The attenuation of the device as the RF signal passes through it can be varied by either an analog (dc voltage) or digital (bits) means. The mixer 12 is used to frequency convert the broadband signal. Output of the mixer will be various combinations of the input signals as is known in the art. A low noise amplifier (LNA) 14 is used to increase the power level of the broadband signal while adding a minimal amount of noise degradation. A power amplifier (PA) 16 is used to increase power level of an RF/microwave signal. The maximum capable output power level achieved with a power amplifier is usually higher than that of a normal amplifier or LNA.
With the example shown in FIG. 1, the attenuator 10 attenuates the input signal to mixer 12, thereby reducing C/N when adjusting power output level down from maximum level. An analog attenuator usually exhibits a nonlinear response over frequency and output level range. This nonlinear response increases calibration time. Further, analog attenuators are difficult to temperature compensate. A digital attenuator is required to have the same output resolution as the transmitter output.
FIG. 2 shows a prior art example of open loop control at a radio frequency (RF) using an analog or digital attenuator 10 after the mixer 12. As discussed above in connection with FIG. 1, an analog attenuator exhibits nonlinear response over frequency and output level range and is difficult to calibrate. Further, analog attenuators are difficult to temperature compensate over frequency. A digital attenuator needs to have the same step size as the required level adjust and must operate over the entire frequency range of operation.
FIG. 3 illustrates a prior art embodiment of open loop control at both IF and RF having a first analog or digital attenuator 10 before the frequency tuned mixer 12 and a second analog or digital attenuator 10 after the frequency tuned mixer. Such a system suffers from the same disadvantages as discussed above in connection with the prior art systems shown in FIGS. 1 and 2, but can be used to divide the adjustment range between two circuits.
FIG. 4 illustrates a prior art closed loop system for RF power control. A feedback loop is provided from a directional coupler 18 to an analog attenuator 10′ which is positioned after the frequency tuned mixer. The directional coupler 18 is used to couple or sample the broadband signal with limited degradation to the main signal path. A detector 20 in the feedback loop detects the power level of the signal and converts it into a dc voltage. A comparator 22 takes the dc voltage from the detector 20 and a reference dc voltage from a digital to analog converter (DAC) 24 as inputs and outputs their difference. The difference value is used to control the second (analog) attenuator 10′. This system requires an analog attenuator 10′ after the mixer 12 that operates over the entire frequency range. This system does not allow for a digital attenuator to be used in a continuous control loop, since a digital attenuator can only be used to make adjustments in discrete step sizes (e.g., 1 dB, 2 dB, etc.). Therefore, a digital attenuator will not follow continuous power changes (e.g., a 0.25 dB of change per minute as heating occurs). Further, such a system requires many frequency band calibrations to obtain required output level accuracy.
It would be advantageous to provide methods and apparatus for optimal control of the output power level of a broadband signal to a high degree of accuracy, while at the same time providing an optimal topology to maintain the output carrier-to-noise ratio of the transmitter.
It would be further advantageous to provide methods and apparatus for accurately calibrating or setting the output power of transmitter in a broadband communication system for any required frequency and output level in a simple manner that requires minimal effort.
The methods and apparatus of the present invention provide the foregoing and other advantages.